Fix existing undefined functions

[bbarker]

Especially instance methods, e.g.:

• mxArraySetOffset = error "mxArraySet undefined for MStruct: use mStructSet" The issue here, and why we can't use arrayDataSetDef, is that MStruct has no instance of Storable - I haven't checked to see if this is entirely possible.

[bbarker]

This was fixed here, but not merged to master yet.

Didn't use Storable but long story short, traversed the fields of input MStructs, thus looping over mStructSet for implementing mxArraySetOffset.